Neural implant for microstimulation

ABSTRACT

This disclosure provides systems and methods for delivering a neural stimulation pulse. A neural implant device can include an energy harvesting circuit configured to receive an input signal and generate an electrical signal based on the received input signal. A diode rectifier in series with the energy harvesting circuit can rectify the electrical signal. The energy harvesting circuit and the diode rectifier can be encapsulated within a biocompatible electrically insulating material. A neural electrode can be exposed through the biocompatible electrically insulating material. The neural electrode can be configured to deliver a neural stimulation pulse. The neural implant device can have a volume that is less than about 1.0 cubic millimeter.

BACKGROUND

Electrical nodes can be implanted into the body of a patient for thepurpose of stimulating nervous tissue. Typically, such devices includewired electrodes that receive energy from a source located outside thebody. Implantable devices that connect to an external power source withwires are very invasive, and can cause long term damage to neuraltissue. Wireless neural implants are available, but are often unable toprovide neural stimulation pulses of sufficient voltage and current toeffectively stimulate nervous tissue. Furthermore, when a plurality ofwireless neural devices are implanted into a patient, it can bedifficult to selectively control the individual devices.

SUMMARY

The systems, methods, and devices of the disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a neural implant device. The neural implant devicecan include an energy harvesting circuit configured to receive an inputsignal and generate an electrical signal based on the received inputsignal. The neural implant device can include a diode rectifier inseries with the energy harvesting circuit. The diode rectifier can beconfigured to rectify the electrical signal. The energy harvestingcircuit and the diode rectifier can be encapsulated within abiocompatible electrically insulating material. The neural implantdevice can include a neural electrode exposed through the biocompatibleelectrically insulating material and configured to deliver a neuralstimulation pulse. The neural implant device can have a volume that isless than about 1 cubic millimeter.

In some implementations, the neural implant device can have a volumethat is less than about 0.5 cubic millimeters. In some implementations,the energy harvesting circuit can include an inductor in parallel with acapacitor. In some implementations, the inductor can have an inductancein the range of about 10 nH to about 500 μH.

In some implementations, the inductor can include a coil made from atleast one of copper, aluminum, silver or gold wound around a ferritecore. The coil can be wound within the biocompatible electricallyinsulating material, and the biocompatible electrically insulatingmaterial can be substantially free from air bubbles. In someimplementations, the biocompatible electrically insulating material caninclude at least one of parylene, silicone, and epoxy.

In some implementations, the inductor can have a diameter that is nogreater than about 0.4 millimeters and a length that is no greater thanabout 1.0 millimeters. In some implementations, the capacitor can have acapacitance in the range of about 1 pF to about 10 nF. In someimplementations, the capacitor and the inductor can form a circuithaving a resonance frequency in the range of about 100 kHz to about 100MHz. In some implementations, at least two of the capacitor, theinductor, and the diode rectifier are included on a single chip. In someimplementations, the energy harvesting circuit can include at least onepiezoelectric energy harvester.

In some implementations, the neural implant device can include an energystorage device configured to store a voltage from an output of the dioderectifier and a first switch configured to discharge the stored voltageto the neural electrode when the switch is in a closed position. In someimplementations, the first switch can include at least one of amicroelectromechanical systems (MEMS) magnetic reed switch or anelectrostatically actuated MEMS switch. In some implementations, thefirst switch can be a normally open switch configured to close inresponse to a wireless actuation signal. In some implementations, thefirst switch can exhibit hysteresis, such that an amplitude of thewireless actuation signal required to close the switch is greater thanan amplitude of the wireless actuation signal required to hold theswitch in a closed position. In some implementations, the wirelessactuation signal can include a DC component selected to be of sufficientto magnitude to hold the switch in a closed position and an AC componenthaving a frequency that matches a resonant frequency of the switch. Thesum of the amplitudes of the DC component and the AC component can besufficient to close the switch.

In some implementations, the neural implant device can include anantenna coupled to the energy harvesting circuit. The antenna can beconfigured to receive the input signal from a transmitter and providethe input signal to the energy harvesting circuit.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a method for stimulating muscle ornervous tissue. The method can include receiving, by an energyharvesting circuit, an input signal. The method can include converting,by the energy harvesting circuit, the input signal to an electricalsignal. The method can include rectifying the electrical signal toprovide a rectified signal to an energy store. The method can includedischarging, by a switch, a stored voltage from the energy store toproduce a neural stimulation pulse.

In some implementations, the method can include receiving, by theswitch, a wireless actuation signal causing the switch to discharge thestored voltage from the energy store. In some implementations, the inputsignal can have a frequency in the range of about 100 kHz to about 100MHz, and the actuation signal can have a frequency in the range of about1 kHz to about 50 kHz. In some implementations, the energy harvestingcircuit can include an inductor in parallel with a capacitor. In someimplementations, the energy harvesting circuit can include at least onepiezoelectric energy harvester.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a system for stimulating muscle ornervous tissue. The system can include a power signal generator and aplurality of implantable devices. Each implantable device can include anenergy harvesting circuit configured to receive an input signal from thepower signal generator and generate an electrical signal based on thereceived input signal. Each implantable device can include a dioderectifier in series with the energy harvesting circuit and configured torectify the electrical signal. The energy harvesting circuit and thediode rectifier can be encapsulated within a biocompatible electricallyinsulating material. Each implantable device can include an energystorage device configured to store a voltage from an output of the dioderectifier and a switch configured to discharge the stored voltage to aneural electrode when the switch is in a closed position. The system caninclude an actuation signal generator configured to generate actuationsignals to selectively close the switches of the plurality ofimplantable devices.

In some implementations, the switch of a first one of the plurality ofimplantable devices can be configured to close in response to anactuation signal that differs from an actuation signal associated withthe switch of at least a second one of the plurality of implantabledevices. In some implementations, the system can include a controllerconfigured to control the actuation signal generator and the powersignal generator.

In some implementations, each of the implantable devices can have avolume that is less than about 0.3 cubic millimeters. In someimplementations, the energy harvesting circuit of each of the pluralityof implantable devices can include an inductor in parallel with acapacitor. In some implementations, at least two of the capacitor, theinductor, the diode rectifier, and the switch can be included on asingle chip. In some implementations, the energy harvesting circuit ofeach of the plurality of implantable devices can include at least onepiezoelectric energy harvester. In some implementations, the switch ofeach of the plurality of implantable devices can include at least one ofa microelectromechanical systems (MEMS) reed switch or anelectrostatically actuated MEMS switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the figures, described herein,are for illustration purposes only. It is to be understood that in someinstances various aspects of the described implementations may be shownexaggerated or enlarged to facilitate an understanding of the describedimplementations. In the drawings, like reference characters generallyrefer to like features, functionally similar elements, and/orstructurally similar elements throughout the various drawings. Thedrawings are not necessarily to scale. Instead, emphasis is placed uponillustrating the principles of the teachings. The drawings are notintended to limit the scope of the present teachings in any way. Thesystem and method may be better understood from the followingillustrative description with reference to the following drawings inwhich:

FIG. 1 shows a schematic diagram of an example neural implant device,according to an illustrative implementation.

FIG. 2 shows a schematic diagram of another example neural implantdevice, according to an illustrative implementation.

FIG. 3 shows a cross-sectional view of a MEMS reed switch that can beused in the neural implant device of FIG. 2, according to anillustrative implementation.

FIG. 4 shows a system including multiple instances of the neural implantdevice shown in FIG. 2, according to an illustrative implementation.

FIG. 5 shows a block diagram of an example method for delivering aneural stimulation pulse, according to an illustrative implementation.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The various concepts introduced above and discussed in greater detailbelow may be implemented in any of numerous ways, as the describedconcepts are not limited to any particular manner of implementation.Examples of specific implementations and applications are providedprimarily for illustrative purposes.

This disclosure is generally related to systems and methods fordelivering a neural stimulation pulse. More particularly, the systemsand methods disclosed provide techniques for wirelessly stimulatingneural tissue in a patient via one or more wireless neural devicesimplanted into the patient. In some implementations, each device can beconfigured to store a voltage, and the voltage can be discharged throughnervous tissue adjacent to the device when a switch located on thedevice is closed. In some implementations, each device can be addressedindividually from the other devices, such that subsets of the devicescan be activated simultaneously.

FIG. 1 shows a schematic diagram of an example neural implant device100, according to an illustrative implementation. The device 100includes an energy harvesting circuit 105 in series with a dioderectifier 110. Electrodes 115 a and 115 b are positioned on either sideof nervous tissue 120, which is modeled as a resistor. The electrodes115 a and 115 b receive an electrical signal from the energy harvestingcircuit 105 and diode rectifier 110, and deliver a neural stimulationpulse to the nervous tissue 120, which may include a muscle, a nerve ora bundle of nerves targeted for stimulation. The device 100 also mayinclude a capacitor 119. The broken line surrounding the capacitor 237indicates that the capacitor 237 is an optional component. A powersignal generator 107 is configured to deliver an input signal to theenergy harvesting circuit 105. The power signal generator 107 may beexternal to the body of the patient.

In the device 100 shown in FIG. 1, the energy harvesting circuit 105 isimplemented as an inductor 125 in parallel with a capacitor 130.Together, the inductor 125 and the capacitor 130 can be configured toreceive the signal from the power signal generator 107, and convert thereceived signal into an electrical signal. For example, in someimplementations the power signal generator 107 can be configured togenerate an input signal in the form of an applied magnetic field. Themagnetic field generated by the power signal generator 107 can beselected to ensure that it can be efficiently transmitted through thebody tissue of the patient to the energy harvesting circuit 105. In someimplementations, the magnetic field generated by the power signalgenerator can have a frequency in the range of about 100 kHz to about100 MHz. The magnetic field induces a variable voltage across theinductor 125, which resonates with the capacitor 130. Thus, the inductor125 and the capacitor 130, which together form the energy harvestingcircuit 105, generate an electrical signal in response to the signalreceived from the power signal generator 107.

In some implementations, the electrical characteristics of the inductor125 and the capacitor 130 can be selected to provide electricalresonance at a frequency matching the frequency of the magnetic fieldgenerated by the power signal generator 107. In some implementations,the capacitor 130 may be a commercially available capacitor. Thecapacitor 130 can have a capacitance in the range of about 1 pF to about10 nF. The inductor 125 can have an inductance in the range of about 10nH to about 500 μH. In some implementations, the inductor 125 can bemade from a coil of conductive material, such as copper, silver,aluminum, gold, or platinum. The coil can be wound around a ferrite corewithin a biocompatible hermetic insulating material, such as silicone,parylene, or epoxy. For example, an insulating material in liquid form,such as liquid silicone, can be dripped onto the inductor as theconductive coil is wrapped around the ferrite core. After the coil hasbeen wound, the liquid insulating material can be cured. This processcan help to reduce the likelihood of developing air bubbles within theinsulating material. In some implementations, the ferrite core may havea diameter in the range of about 0.1 millimeters to about 0.3millimeters. In some implementations, the inductor can have a diameterof less than about 0.6 millimeters, less than about 0.5 millimeters,less than about 0.4 millimeters, less than about 0.3 millimeters, orless than about 0.2 millimeters. The inductor can have a length of lessthan about 1.2 millimeters, less than about 1.1 millimeters, less thanabout 1 millimeter, less than about 0.9 millimeters, or less than about0.8 millimeters.

Because the device 100 is implanted into a patient, smaller sizes forindividual components may be preferable in order to minimize healthrisks and discomfort to the patient. For example, in someimplementations, the entire implantable device 100 can have a volume ofless than about 1 cubic millimeter, less than about 0.5 cubicmillimeters, less than about 0.4 cubic millimeters, 0.3 cubicmillimeters, or less than about 0.2 cubic millimeters. In someimplementations, the implantable device 100 can have a volume of in therange of about 0.1 cubic millimeters to about 0.5 cubic millimeters. Insome implementations, the device 100 can be generally cylindrical inshape, and may have a diameter in the range of about 0.3 millimeters toabout 0.7 millimeters and a length in the range of about 1.3 millimetersto about 1.7 millimeters. In some implementations, the device may have adiameter of less than about 0.5 millimeters and a length of less thanabout 1.5 millimeters. In some other implementations, the device 100 mayhave another shape, such as a cube or a rectangular prism. In stillother implementations, the device 100 may have an irregular shape.

It should be noted that, while FIG. 1 depicts the energy harvestingcircuit 105 as an inductor 125 in parallel with a capacitor 130, theenergy harvesting circuit 105 may also be implemented in other ways. Forexample, the energy harvesting circuit 105 can include a piezoelectricenergy harvester. In general, a piezoelectricity is electrical chargethat accumulates in a material in response to mechanical stress. Thus, apiezoelectric energy harvester can be configured to respond to a signalgenerated by the power signal generator 107 in a way that inducesmechanical stress in the piezoelectric energy harvester, which in turncauses the piezoelectric energy harvester to generate an electricalcharge. In some implementations, the energy harvesting circuit 105 canbe implemented as a block of PZT, a piezoelectric microshell transducer,a piezoelectric bimorph, a piezoelectric monomorph, or any other type ofpiezoelectric energy harvesting device capable of generating anelectrical signal in response to an input signal received from the powersignal generator 107. In such implementations, the power signalgenerator 107 can be configured to produce a signal that inducesmechanical stress in the piezoelectric energy harvester. For example,the piezoelectric energy harvester can be or can include an ultrasonictransducer, and the power signal generator 107 can generate an acousticsignal to be received by the piezoelectric energy harvester. Theacoustic signal can induce stress in the piezoelectric energy harvester,which can then be converted into an electrical signal. Other types ofenergy harvesting devices also may be used as the energy harvestingcircuit 105. For example, the energy harvesting circuit 105 can be orcan include a photovoltaic energy harvester, a pyroelectric energyharvester, a thermoelectric energy harvester, or an electrostatic energyharvester.

Furthermore, the signal generated by the power signal generator 107 canbe selected based in part on the characteristics of the components usedto form the energy harvesting circuit 105. In some implementations, thesignal generated by the power signal generator may include a near-fieldsignal. In some other implementations, the signal generated by the powersignal generator may include a far-field signal. In someimplementations, the device 100 may include an antenna coupled to theenergy harvesting circuit. The antenna can be configured to receive thesignal from the power signal generator 107 and deliver the signal to theenergy harvesting circuit 105.

As discussed above, the energy harvesting circuit 105 can be configuredto convert a time-varying input signal from the power signal generator107 into a time-varying electrical signal. In some implementations, thefrequencies that are most efficient for transmitting the input signal tothe device 100 through the body of a patient may be significantly higherthan the frequencies of neural stimulation pulses that most efficientlyexcite the nervous tissue 120. As a result, the frequency of theelectrical signal generated by the energy harvesting circuit 105 may betoo high to efficiently stimulate nervous tissue 120, because thefrequency of the electrical signal produced by the energy harvestingcircuit 105 will be approximately equal to the frequency of the signalreceived from the power signal generator 107. To address this problem,the diode rectifier 110 is positioned in series with the energyharvesting circuit 105. The diode rectifier 110 receives the electricalsignal from the energy harvesting circuit 105 and produces a rectifiedsignal having a lower frequency (i.e., a low frequency moving averagevoltage proportional to the amplitude of the RF electrical signalreceived from the energy harvesting circuit 105). This lower frequencyrectified signal is then delivered to the nervous tissue 120 via theelectrodes 115 a and 115 b.

In some implementations, the electrical signal produced by the energyharvesting circuit 105 can have a relatively small magnitude. Therefore,the diode rectifier 110 may be selected to have a relatively low turn-onvoltage and a low parasitic capacitance in order to efficiently convertthe electrical signal from the energy harvesting circuit 105 to arectified signal. For example, in some implementations the dioderectifier 110 may have a turn-on voltage of less than about 0.3 volts,less than about 0.2 volts, less than 0.1 volts, or less than about 0.05volts. Example devices that may be suitable for use as the dioderectifier 110 can include an RF Schottky diode or an RF detector diode.In some implementations, including the optional capacitor 119 in thedevice 100 can help to turn on the diode at lower voltages. In someimplementations, the capacitor 119 may have a capacitance in the rangeof about 100 pF to about 10 nF.

In some implementations, the electrodes 115 a and 115 b may be exposedto the nervous tissue 120 through an encapsulating layer that surroundsthe other components of the implantable device 100. For example, theenergy harvesting circuit 105 and the diode rectifier 110 can beencapsulated in a biocompatible material, such as parylene, silicone,epoxy, or any other non-toxic insulating material. The electrodes 115 aand 115 b can be electrically connected to the energy harvesting circuit105 and the diode rectifier 110 as shown in the schematic of FIG. 1, butmay be exposed through biocompatible encapsulation material tofacilitate delivery of the neural stimulation pulse to the nervoustissue 120. The electrodes 115 a and 115 b can be formed from anynon-toxic conductive material. In some implementations, the electrodes115 a and 115 b may be formed from porous platinum. In some otherimplementations, the electrodes 115 a and 115 b may be formed from gold.

FIG. 2 shows a schematic diagram of another example neural implantdevice 200, according to an illustrative implementation. The device 200includes many of the features discussed above in connection with thedevice 100 shown in FIG. 1, and like reference numerals refer to likeelements. For example, the device 200 includes an energy harvestingcircuit 205, which in this implementation is formed from an inductor 225in parallel with a capacitor 230. The energy harvesting circuit 205receives a signal from a power signal generator 207, and generates anelectrical signal that is rectified by the diode rectifier 210, which ispositioned in series with the energy harvesting circuit 205. Inaddition, the device 200 includes a charge storage capacitor 235, aswitch 203, and an actuation signal generator 209. The device 200 mayalso include a capacitor 237. The broken line surrounding the capacitor237 indicates that the capacitor 237 is an optional component.

The charge storage capacitor 235 is configured to store electricalcharge that exists across the charge storage capacitor 235 in responseto the rectified signal produced by the diode rectifier 210. When theswitch 203 is open, charge stored by the charge storage capacitor 235has no path through which to be discharged. As a result, charge willaccumulate across the charge storage capacitor 235 while a rectifiedsignal is present at the output of the diode rectifier 210, and thecharge storage capacitor 235 will retain the accumulated charge untilthe switch 203 is closed. It should be noted that, while a chargestorage capacitor 235 is shown in FIG. 2, in some implementations,another charge storage element could be substituted. For example, arechargeable battery could be used as a charge storage element in thedevice 200 instead of the charge storage capacitor 235.

When the switch 203 is closed, the charge stored by the charge storagecapacitor 235 can be discharged through the nervous tissue 220 via theexposed electrodes 215 a and 215 b. Thus, a neural stimulation pulse isonly provided to the nervous tissue 220 when the switch 203 is closed.By selectively opening and closing the switch 203, a selected pattern ofneural stimulation pulses can be delivered to the nervous tissue 220. Insome implementations, the actuation signal generator 209 can beconfigured to selectively open and close the switch 203 to generate sucha pattern of neural stimulation pulses. For example, the switch 203 canbe a normally open switch that is configured to close only in responseto an applied actuation signal, which may be a wireless signal. Theactuation signal generator 209 can transmit an actuation signal thatcauses the switch 203 to close temporarily, thereby allowing the voltagestored by the charge storage capacitor 203 to be discharged through thenervous tissue 220. One example implementation of the switch 203 isdiscussed further below in connection with FIG. 3.

In some implementations, the addition of the charge storage capacitor235 and the switch 203 may counterintuitively allow the size of thedevice 200 to be reduced relative to the size of the device 100 shown inFIG. 1, which does not include these components. For example, attemptingto reduce the sizes of the wires and components of the device 100 shownin FIG. 1 results in increased electrical resistance, and decreasedinstantaneous power of the neural stimulation pulse received by thenervous tissue 120. Incorporating the charge storage capacitor 235allows for the accumulation of a larger amount of electrical energy inthe device 200, which can be discharged through the tissue 220 in ashort pulse by momentarily closing the switch 203. Thus, the device 200can be capable of transmitting short pulses at a higher average powerthan the average power of the neural stimulation signal produced by thedevice 100 shown in FIG. 1. As a result, the components of the device200 can be made smaller and the device 200 can still be capable ofdelivering neural stimulation pulses of sufficient power, even whentaking into account the increased electrical resistance that may occurdue to the relatively smaller components.

In some implementations, the switch 203 can be configured to close for aduration of time equal to a desired duration of the neural stimulationpulse. For example, in some implementations, the desired duration of theneural stimulation pulse may be about 0.1 milliseconds, about 0.2milliseconds, about 0.3 milliseconds, or about 0.4 milliseconds. Theneural stimulation pulse may have a charge output (i.e., an integratedcurrent) in the range of about 5 nC to about 15 nC and a voltage in therange of about 0.5 volts to about 1.5 volts. In some implementations,the neural stimulation pulse may have a charge output of about 10 nC anda voltage of about 1 volt.

The device 200 also can be individually activated independently of othersuch devices 200 that may also be implanted within the patient. Forexample, the switch 203 can be configured to respond to an actuationsignal that differs from an actuation signal associated with the switchof another implantable device. Thus, multiple instances of the device200 can be implanted in the patient, and each can be activatedindependently to achieve a desired therapeutic effect. In someimplementations, a group of two or more devices 200 may be configured torespond to the same actuation signal, such that the group of devices 200can be activated simultaneously. Other devices 200 not in the group maybe configured to respond to a different actuation signal, so thatactivation of the devices within the group can be achieved independentlyof activation of the devices not within the group. An example of asystem including multiple instances of the device 200 is describedfurther below in connection with FIG. 4.

In some implementations, the optional capacitor 237 may be added to thesystem 200 to cause the integrated average current through thestimulation electrodes 215 a and 215 b to be zero. Zero average DCcurrent can help to preserve the impedance properties of some types ofneural electrodes, and also may reduce corrosion, pH changes, and localtissue damage as well.

In some implementations, various components of the device 200 may beincluded on a single chip, which can help to reduce the overall size ofthe device 200. For example, in some implementations, the dioderectifier 210 and one or both of the capacitors 235 and 237 may beincluded on a single chip. In general, any two or more of the dioderectifier 210, the inductor 225, the capacitor 230, the capacitor 235,and the capacitor 237 may be included on a single chip.

FIG. 3 shows a cross-sectional view of a microelectromechanical systems(MEMS) reed switch 300 that can be used in the neural implant device ofFIG. 2, according to an illustrative implementation. For example, theMEMS reed switch 300 can serve as the switch 203 shown in FIG. 2. TheMEMS reed switch 300 is a single-pole, single-throw switch including asubstrate 305, a first contact 310, and a second contact 315. Thesubstrate 305 is formed from a non-magnetic material, while the firstcontact 310 and the second contact 315 are formed from a magneticmaterial, such as an iron-nickel alloy. Alternatively, the switch may beelectrostatically actuated by the voltage stored on capacitor 235,discharging a pulse whenever the voltage across the switch 203 reaches alevel sufficient to actuate the switch.

As shown, the second contact 315 is fixed to the substrate 305 at oneend, and the other end of the second contact 315 is suspended above thefirst contact 310 such that the MEMS reed switch 300 is normally open.When a magnetic field of sufficient amplitude along the long directionof the switch is applied in the vicinity of the MEMS reed switch 300 ata frequency that matches a resonant frequency of the second contact 315,the second contact 315 bends downwards to touch the first contact 310,thereby closing the switch 300. In some implementations, when the MEMSreed switch 300 serves as the switch 203 shown in FIG. 2, the firstcontact 310 can be electrically connected to the output of the dioderectifier 210 and the second contact can be electrically connected tothe electrode 215 a, as shown in the schematic of FIG. 2. In someimplementations, the MEMS reed switch 300 may be configured to close inresponse to a magnetic field having a frequency that differs from afrequency associated with a different MEMS reed switch 300. Thisconfiguration can allow multiple instances of the MEMS reed switch 300to be actuated independently. In some implementations, the MEMS reedswitch 300 can exhibit hysteresis, such that it closes at a higheramplitude magnetic field than is required for it to remain closed. Insuch implementations, the MEMS reed switch 300 may be operated with acombined DC plus AC magnetic field. The DC field can be selected to bestrong enough to hold the MEMS reed switch 300 in a closed position, andthe AC field can be tuned to the resonant frequency of selectedstimulation nodes.

The MEMS reed switch 300 also can be selected to have a resonantfrequency different from the resonant frequency of the generated by thepower signal generator 207 shown in FIG. 2, so that the MEMS reed switch300 will not accidentally close in response to the signal generated bythe power signal generator 207. As discussed above, the signal generatedby the power signal generator 207 may be in the range of about 100 kHzto about 100 MHz. In some implementations, the resonant frequency of theMEMS reed switch 300 may be significantly lower than the frequency ofthe signal generated by the power signal generator 207. For example, theresonant frequency of the MEMS reed switch 300 may be in the range ofabout 1 kHz to about 50 kHz. In some implementations, the resonantfrequency of the MEMS reed switch 300 may be about 10 kHz. The frequencyof the signal generated by the actuation signal generator 209 shown inFIG. 2 can be selected to match the resonant frequency of the MEMS reedswitch 300. Thus, in some implementations, the frequency of the signalgenerated by the actuation signal generator 209 may be in the range ofabout 1 kHz to about 50 kHz.

It should be noted that the MEMS reed switch 300 is illustrative only,and in some implementations, the switch 203 may be implemented as adifferent type of switch. For example, the switch 203 may instead beimplemented as an electrostatic relay configured to close at a thresholdvoltage. Thus, when the voltage across the charge storage capacitor 235shown in FIG. 2 exceeds the threshold voltage associated with theelectrostatic relay, the electrostatic relay will close, which allowsthe charge storage capacitor 235 to be discharged to produce a neuralstimulation pulse. After the charge storage capacitor 235 is discharged,the voltage across the charge storage capacitor will again fall belowthe threshold voltage, and the electrostatic relay will open.

FIG. 4 shows a system 400 including multiple instances of the neuralimplant device shown in FIG. 2, according to an illustrativeimplementation. As shown, the system 400 includes implantable devices200 a-200 n (generally referred to as devices 200), each of which isassociated with a respective portion of nervous tissue 220 a-220 n(generally referred to as nervous tissue 220). A power signal generator207 and an actuation signal generator 209 communicate with each of thedevices 200. A controller 410 communicates with each of the power signalgenerator 207 and the actuation signal generator 209.

The system 400 can include any number of devices 200. In someimplementations, the system 400 may include tens, hundreds, or thousandsof the devices 200, all of which may be implanted into a patient. Insome implementations, all of the devices 200 may be implanted near asingle target muscle, nerve, brain structure, or other nervous tissue.In some other implementations, a subset of the devices 200 may beimplanted near a first targeted portion of muscle, nervous tissue, andanother subset of the devices 200 may be implanted near a secondtargeted portion of nervous tissue. In some implementations, some or allof the devices 200 may be implanted substantially simultaneously. Forexample, a plurality of devices 200 may be loaded into a syringe, andinjected into the patient near the targeted muscle or nervous tissue. Insome other implementations, the devices 200 may each be implanted withinthe patient separately.

The devices 200 may be used to treat any condition that can benefit fromneural stimulation. For example, the devices 200 may be implanted in thebrain for cortical or deep-brain stimulation. In some otherimplementations, the devices 200 may be implanted into peripheralnerves. Stimulation of peripheral nerves can help to treat conditionssuch as chronic pain, high-blood pressure, or hormonal imbalance.Additionally, using a plurality of devices 200 as shown in FIG. 4,rather than a single device 200, can help to improve the efficacy ofneural stimulation treatment. The devices 200 may be used to stimulatemuscles to compensate for damaged or severed nerves, as part of a systemto restore motion to a patient.

The devices 200 can be individually activated to achieve a desiredtherapeutic effect. For example, it may be desirable to activate onlythe subset of devices 200 that are positioned near a first targetedportion of nervous tissue without activating other devices 200 that maybe implanted elsewhere. As discussed above in connection with FIG. 2,each device 200 can be activated by closing the switch 203 that allowsthe charge storage capacitor 235 to be discharged. By appropriatelyselecting switches 203 that respond to different actuation signals, thesystem 400 can be configured such that a desired subset of the devices200 can be activated simultaneously without activating the devices 200that are not part of the desired subset. Therefore, all of the devices200 that are implanted near the first targeted portion of nervous tissuemay include switches 203 that respond to an actuation signal having aparticular frequency. The controller 410 can cause all of the devices200 near the first targeted portion of nervous tissue to generate neuralstimulation pulses substantially simultaneously by controlling theactuation signal generator 209 to generate an actuation signal with afrequency that matches the frequency associated with switches 203 of thedevices 200 that are positioned near the first targeted portion ofnervous tissue. Other devices 200 that are not intended to stimulate thefirst targeted portion of nervous tissue may include switches thatactuate in response to an actuation signal having a different frequency.

The above configuration can make use of a simple power signal generator207, because the triggering of the neural stimulation pulses is managedindependently of the energy harvesting. As a result, the power signalgenerator can emit a constant input signal configured to be convertedinto an electrical signal by all of the devices 200, even during periodsin which it is desired that some of the devices 200 do not deliver aneural stimulation pulse. All of the devices 200 can thereforesimultaneously harvest energy from the signal provided by the powersignal generator 207. The controller 410 can cause the energy stored bythe devices 200 to be selectively discharged to produce neuralstimulation pulses by commanding the actuation signal generator 209 toproduce actuation signals according to a desired neural stimulationscheme.

FIG. 5 shows a block diagram of an example method 500 for delivering aneural stimulation pulse, according to an illustrative implementation.In brief overview the method 500 includes receiving a an input signal(stage 502). The input signal is converted to an electrical signal(stage 504). The electrical signal is rectified to provide a rectifiedsignal to a charge storage element (stage 506). A voltage is dischargedfrom the charge storage element to produce a neural stimulation pulse(stage 508).

In some implementations, the method 500 can be carried out by theimplantable neural stimulation device 200 shown in FIG. 2. For example,with reference to FIGS. 2 and 5, the energy harvesting circuit 205 canreceive an input signal from a power signal generator 207 (stage 502).The input signal can be, for example, a magnetic signal whose frequencymatches the resonant frequency of the inductor 225 and the capacitor 230that form the energy harvesting circuit 205. As a result of theresonance, the energy harvesting circuit 205 can convert the inputsignal received from the power signal generator 207 into an electricalsignal (stage 504). The diode rectifier 210 can receive the electricalsignal from the energy harvesting circuit 205, and can rectify thesignal to provide a rectified signal to the charge storage capacitor 235(stage 506). The rectified signal causes charge to accumulate across thecharge storage capacitor 235. By selectively closing the switch 203, thevoltage stored by the charge storage capacitor can be discharged toproduce a neural stimulation pulse that is delivered to the nervoustissue 220 via the electrodes 215 a and 215 b (stage 508).

Various modifications to the implementations described in thisdisclosure may be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. Thus, the claims are not intended to be limited to theimplementations shown herein, but are to be accorded the widest scopeconsistent with this disclosure, the principles and the novel featuresdisclosed herein.

Certain features that are described in this specification in the contextof separate implementations also can be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation also can be implemented inmultiple implementations separately or in any suitable subcombination.Moreover, although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Further, the drawings may schematically depict one more exampleprocesses in the form of a flow diagram. However, other operations thatare not depicted can be incorporated in the example processes that areschematically illustrated. For example, one or more additionaloperations can be performed before, after, simultaneously, or betweenany of the illustrated operations. In certain circumstances,multitasking and parallel processing may be advantageous. Moreover, theseparation of various system components in the implementations describedabove should not be understood as requiring such separation in allimplementations. Additionally, other implementations are within thescope of the following claims. In some cases, the actions recited in theclaims can be performed in a different order and still achieve desirableresults.

What is claimed is:
 1. A neural implant device, comprising: an energyharvesting circuit configured to receive an input signal and generate anelectrical signal based on the received input signal; a diode rectifierin series with the energy harvesting circuit, the diode rectifierconfigured to rectify the electrical signal, wherein the energyharvesting circuit and the diode rectifier are encapsulated within abiocompatible electrically insulating material; a neural electrodeexposed through the biocompatible electrically insulating material, theneural electrode configured to deliver a neural stimulation pulse;wherein the neural implant device has a volume that is less than about 1cubic millimeter.
 2. The neural implant device of claim 1, wherein theneural implant device has a volume that is less than about 0.5 cubicmillimeters.
 3. The neural implant of claim 1, wherein the energyharvesting circuit comprises an inductor in parallel with a capacitor.4. The neural implant of claim 3, wherein the inductor has an inductancein the range of about 10 nH to about 500 μH.
 5. The neural implant ofclaim 3, wherein the inductor comprises a coil made from at least one ofcopper, aluminum, silver or gold wound around a ferrite core, whereinthe coil is wound within the biocompatible electrically insulatingmaterial, and the biocompatible electrically insulating material issubstantially free from air bubbles.
 6. The neural implant of claim 1,wherein the biocompatible electrically insulating material comprises atleast one of parylene, silicone, and epoxy.
 7. The neural implant ofclaim 3, wherein the inductor has a diameter that is no greater thanabout 0.4 millimeters and a length that is no greater than about 1.0millimeters.
 8. The neural implant of claim 3, wherein the capacitor hasa capacitance in the range of about 1 pF to about 10 nF.
 9. The neuralimplant of claim 3, wherein the capacitor and the inductor form acircuit having a resonance frequency in the range of about 100 kHz toabout 100 MHz.
 10. The neural implant of claim 3, wherein at least twoof the capacitor, the inductor, and the diode rectifier are included ona single chip.
 11. The neural implant of claim 1, wherein the energyharvesting circuit comprises at least one piezoelectric energyharvester.
 12. The neural implant of claim 1, further comprising: anenergy storage device configured to store a voltage from an output ofthe diode rectifier; and a first switch configured to discharge thestored voltage to the neural electrode when the switch is in a closedposition.
 13. The neural implant of claim 12, wherein the first switchcomprises at least one of a microelectromechanical systems (MEMS)magnetic reed switch or an electrostatically actuated MEMS switch. 14.The neural implant of claim 12, wherein the first switch is a normallyopen switch configured to close in response to a wireless actuationsignal.
 15. The neural implant of claim 14, wherein the first switchexhibits hysteresis, such that an amplitude of the wireless actuationsignal required to close the switch is greater than an amplitude of thewireless actuation signal required to hold the switch in a closedposition.
 16. The neural implant of claim 15, wherein the wirelessactuation signal comprises: a DC component selected to be of sufficientto magnitude to hold the switch in a closed position; and an ACcomponent having a frequency that matches a resonant frequency of theswitch, wherein the sum of the amplitudes of the DC component and the ACcomponent is sufficient to close the switch.
 17. The neural implant ofclaim 1, further comprising an antenna coupled to the energy harvestingcircuit, wherein the antenna is configured to: receive the input signalfrom a transmitter; and provide the input signal to the energyharvesting circuit.
 18. A method for stimulating muscle or nervoustissue, the method comprising: receiving, by an energy harvestingcircuit, an input signal; converting, by the energy harvesting circuit,the input signal to an electrical signal; rectifying the electricalsignal to provide a rectified signal to an energy store; anddischarging, by a switch, a stored voltage from the energy store toproduce a neural stimulation pulse.
 19. The method of claim 18, furthercomprising receiving, by the switch, a wireless actuation signal causingthe switch to discharge the stored voltage from the energy store. 20.The method of claim 19, wherein the input signal has a frequency in therange of about 100 kHz to about 100 MHz, and the actuation signal has afrequency in the range of about 1 kHz to about 50 kHz.
 21. The method ofclaim 18, wherein the energy harvesting circuit comprises an inductor inparallel with a capacitor.
 22. The method of claim 18, wherein theenergy harvesting circuit comprises at least one piezoelectric energyharvester.
 23. A system for stimulating muscle or nervous tissue, thesystem comprising: a power signal generator; a plurality of implantabledevices each including: an energy harvesting circuit configured toreceive an input signal from the power signal generator and generate anelectrical signal based on the received input signal; a diode rectifierin series with the energy harvesting circuit, the diode rectifierconfigured to rectify the electrical signal, wherein the energyharvesting circuit and the diode rectifier are encapsulated within abiocompatible electrically insulating material; an energy storage deviceconfigured to store a voltage from an output of the diode rectifier; anda switch configured to discharge the stored voltage to a neuralelectrode when the switch is in a closed position; and an actuationsignal generator configured to generate actuation signals to selectivelyclose the switches of the plurality of implantable devices.
 24. Thesystem of claim 23, wherein the switch of a first one of the pluralityof implantable devices is configured to close in response to anactuation signal that differs from an actuation signal associated withthe switch of at least a second one of the plurality of implantabledevices.
 25. The system of claim 23, further comprising a controllerconfigured to control the actuation signal generator and the powersignal generator.
 26. The system of claim 23, wherein each of theimplantable devices has a volume that is less than about 0.3 cubicmillimeters.
 27. The system of claim 23, wherein the energy harvestingcircuit of each of the plurality of implantable devices comprises aninductor in parallel with a capacitor.
 28. The system of claim 27,wherein at least two of the capacitor, the inductor, the dioderectifier, and the switch are included on a single chip.
 29. The systemof claim 23, wherein the energy harvesting circuit of each of theplurality of implantable devices comprises at least one piezoelectricenergy harvester.
 30. The system of claim 23, wherein the switch of eachof the plurality of implantable devices comprises at least one of amicroelectromechanical systems (MEMS) reed switch or anelectrostatically actuated MEMS switch.